Compound Urban Flooding: The Emerging Hazard for Large Metropolitan Areas



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Flooding is one of the major natural disasters around the globe with wide-reaching impacts on the environment, economy, infrastructure, and human lives. In recent years, compound floods resulting from the co-occurrence of multiple flood drivers like riverine flow, storm surges, sea-level rise, and extreme rainfall have impacted several cities across the United States (US). Metropolitan areas, located in the coastal regions of the US, have become increasingly vulnerable to such flooding conditions due to their unique exposure to multiple flood hazards and population density, rapid urbanization, proximity to major rivers or lakes, and the probable risk of sea level rise. The goal of this study is to identify the major flood drivers impacting metropolitan areas in large tidal estuaries and to quantify their impacts on compound flooding. Washington, DC, and the surrounding communities in Maryland and Virginia is used as an example, because like many US coastal areas, it is often exposed to various flood hazards that overlap spatially and temporally, leading to the potential of compound flooding conditions. Results from the historical data analysis (1931 to 2019) provided strong evidence that compound flooding in the region can be caused by a combination of storm surge or high coastal water levels, riverine flow, local wind, and urban runoff. These events had the highest duration and magnitude during the study period. Among the four Major flood events during this time, three were a result of compound flooding, and only one resulted from high riverine flow. Storm-surge driven coastal floods were more common in Washington, DC compared to river floods when the flood stages were either at Moderate or Minor stage. The local wind with speed above 5.5 m/s and urban runoff also plays a significant role impacting flood levels in the area. The variability in compound flooding extent and depth was also investigated using a 2D hydrodynamic model. Results suggested that locations around the study area can be locally divided into three zones based on the impact of flow and surges: highly riverine flow dominated upstream zone, transition zone with impact from both flow and surges, and coastal water level dominated lower zone. Flood depths during surge dominated events were significantly higher than flow dominated events, almost doubling in some locations towards downstream of the river. Small urban streams in the area were significantly impacted by flow, surge, and rainfall. Low gradient streams were more impacted by compound flooding compared to steep streams. Inundation results showed that cities like Washington, DC will face increased flooding in the long-term future because of sea level rise (SLR), particularly during surge dominated flood events. Finally, the comprehensive analysis of the compound flooding in the region has led to the development of a real-time flood forecast system for compound urban flooding. The spatial and temporal distribution of flooding was significantly captured by the forecast system. This study has provided scientific insights into the physical characteristics and spatial variability of the compound flooding in metropolitan areas along the estuaries and implemented the outcomes of historical data analysis for flood inundation modeling and developing a real-time forecast system for urban areas.